A common treatment for patients with low back pain (LBP) or lower extremity pain is the flexion-distraction therapy, which is a widely used form of chiropractic treatment in which non-impulsive loads are delivered at slow speeds. This therapy is hypothesized to result in motions of lumbar spine segments that increase the intervertebral foraminal space. The proposed study addresses the following research questions: (I). Does the flexion-distraction procedure increase the intervertebral foramina space in the lumbar spine in vitro ? (II). How reproducible are the biomechanical effects of the flexion-distraction therapy. That is, what is the intra- and inter-clinician reliability of the biomechanical parameters while administering this procedure in vitro? (III). What are the loads on the internal tissues of the spinal segments L4-L5 and L5-S1, and are the tissues at risk of injury during the flexion-distraction procedure in vitro? (IV). Is there in vivo trunk muscle activity that may alter the transmission of loads and motion to the spinal segment? In vitro experiments will be conducted on 10 unembalmed whole cadavers to quantitatively describe the three-dimensional displacements, dimensional (height, width and area) changes in the intervertebral foramina, and loads applied to the subject during the flexion-distraction therapeutic procedure for L4-L5 and L5-S1 spinal motion segments. These measurements will utilize an optoelectronic three-dimensional motion analysis system (OPTOTRAK), a three-dimensional digitizer, three-dimensional force plates and a load cell. The intra- and inter-clinician reliability of the measured biomechanical effects of this procedure will be determined based on data collected while three different clinicians administer the procedure three times on each of the 10 cadavers. A computer model developed by the investigators will be used to estimate the internal loads on the ligaments, and the intervertebral disc of the L4-L5 and L5-S1 joints under in vitro conditions of this therapeutic procedure. We will compare these loads with the failure loads of the tissues to assess the risk of injury. Finally, we will conduct in vivo experiments on 60 patients with a history of LBP at L4 to S1 spinal levels. We will quantitatively describe the loads applied to the subjects and the electromyographic activity of six superficial muscles surrounding the lumbar spine during the flexion-distraction therapeutic procedure. The proposed study will be undertaken in collaboration with investigators from the Department of Orthopedic Surgery. Loyola University Stritch School of Medicine, Maywood, Illinois and a secondary care facility, The Hinsdale Orthopedic Clinic.